This invention relates to the preparation of copolyestercarbonates, and more particularly to an improved method for their preparation by interfacial methods.
The preparation of copolyestercarbonates by the interfacially conducted condensation of dihydroxyaromatic compounds with dicarboxylic acids and phosgene in an immiscible aqueous-organic medium is known. It ordinarily takes place in the presence of a tertiary amine, a phase transfer catalyst or both. The product copolyestercarbonates have properties similar to those of polycarbonates but are generally more ductile, especially when the ester units are derived from aliphatic acids.
Among the uses presently under study for copolyestercarbonates is in optical disk fabrication. For this purpose, it is frequently advantageous to incorporate structural units which have very low or even negative intrinsic birefringence, such as those derived from spiro(bis)indane bisphenols and especially 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1 '-spiro(bis)indane (CAS #1568-80-5), hereinafter designated "SBI". When SBI units are present in polycarbonates, however, they are very brittle, processing is difficult and it is almost essential to improve ductility and processability by incorporation of ester units.
The interfacial preparation of copolyestercarbonates is also known. In this method of preparation, reaction is conducted between phosgene and a mixture of a dihydroxyaromatic compound and a dicarboxylic acid (or corresponding acid chloride) in an immiscible aqueous-organic mixture. The reaction takes place under alkaline conditions and in the presence of a catalyst, typically a tertiary amine, phase transfer catalyst or combination of the two.
The polymer ideally produced by this method contains ester and polycarbonate units. It is often found, however, that said polymer also contains moieties incorporating anhydride linkages, which are thermally and hydrolytically unstable.
It is frequently found that anhydride linkages are produced in substantial proportions under normal interfacial reaction conditions. The presence of such linkages can often be minimized by careful control of the pH of the reaction mixture. This has not been found to be true, however, when a large proportion of acid-derived groups, for example 10 mole percent or higher, is desired in the polymer.
U.S. Pat. Nos. 5,510,449 and 5,519,105 describe procedures for the preparation of polycarbonates, including copolyestercarbonates, in which a mixture of phase transfer catalyst and tertiary amine is employed. There is no suggestion, however, of a method for decreasing the proportion of anhydride linkages in a copolyestercarbonate.
Therefore, it is of interest to adjust the reaction conditions in interfacial copolyestercarbonate preparation to suppress the formation of anhydride linkages, particularly when large percentages of ester groups are desired in the polymer.